Porcine reproductive and respiratory syndrome (PRRS) is a chronic viral disease of pigs worldwide. PRRS is endemic in most pork-producing countries, and it is responsible for major economic losses to the swine industry, with an estimated annual loss of $664 million in the US (8).
Since the late 1990s, modified live PRRSV (PRRS-MLV) and killed virus vaccines have been available to control the disease, but neither of them protects pigs completely against heterologous field viruses (27). Like the field virus, PRRS-MLV also induces immunosuppression (29, 30). Moreover, there are several reports of reversion of vaccine virus into virulence leading to severe disease outbreaks (31-34). Although killed PRRSV vaccines are safe, they are poorly immunogenic (35, 36).
Clinical signs of PRRS comprise respiratory and reproductive dysfunction and the causal agent is PRRS virus (PRRSV) (28). PRRSV establishes disease by modulating the pig immune system from as early as two days and continues for several weeks post-infection (14, 15). A particular challenge presented by PRRSV is the immunosuppression caused by PRRSV attributed to virus mediated reduction in production of important cytokines (IFN-α, IFN-γ, and TNF-α), associated with increased secretion of interleukin (IL)-10 and transforming growth factor-β (TGF-β), and upregulation of Foxp3+ T-regulatory cell (Tregs) population (14). In addition, in infected pigs, virus neutralizing (VN) antibodies appear delayed (3-4 weeks) and also their levels remain low (86). Thus, there remains a great need in the art to provide for safe and effective protection and treatment of PRRS.